A conventional vehicle position detecting system uses position indicating information received from artificial satellites to detect the present position of a vehicle. Artificial satellites of global positioning system (GPS) are generally used as the artificial satellites. Position coordinate data and time data of each GPS artificial satellite are successively transmitted as position indicating information from the artificial satellite concerned. In-vehicle equipment receives data from four or more GPS artificial satellites, and the coordinate at which the vehicle is located at present is calculated based on the difference in propagation time in which the data transmitted from each GPS artificial satellite propagates to the vehicle.
The present position determined by this technique may remarkably degraded in precision in accordance with environmental conditions such as a case where obstacles exist around the vehicle and data can be received from only three or less GPS artificial satellites, a case where the vehicle is located at a position at which reflecting structures such as buildings, etc. exist around the vehicle and data from one GPS artificial satellite is received through plural paths, that is, multi-path occurs, or the like.
A map matching correcting technique (for example, EP 0875877 B1, JP 10-307037A) is known as a technique of detecting the accurate present position even under such a condition as described above. According to this technique, the travel path of the vehicle is created and it is matched with roads on map data to correct the present position.
Map data are needed to perform the map matching correction. However, the map data are generally expensive, and thus a position detecting device by which the accurate present position can be detected by performing the map matching correction has been expensive.